The continuing spread of HIV/AIDS in people is predominantly fueled by sexual exposure to HIV-contaminated semen/seminal plasma (SP). SP harbors a variety of pro-inflammatory factors that may facilitate HIV transmission by promoting the production of cytokines/chemokines that recruit permissive cells, enhance the translocation of HIV across the genital epithelium, and activate HIV gene transcription. The levels of these factors have been shown to be altered during HIV infection, with HIV-infected individuals generally harboring higher levels of various pro-inflammatory cytokines in SP. We and others have extensively studied the properties of SP constituents, both with regards to their ability to promote HIV infection and their ability to create an inflammatory state favoring HIV replication. However, one class of factors in SP that has received little attention is exosomes, small extracellular vesicles of which trillions are typically present in a single ejaculate. Exosomes are important intercellular signal transducers and induce various inflammatory or immunomodulatory responses. Although numerous studies have characterized plasma-derived exosomes from both uninfected and HIV-infected individuals, very little has been done to understand the relationship between semen exosomes and HIV transmission. In this R21, we test the hypotheses that HIV infection alters the composition of exosomes in SP and that SP exosomes from HIV-infected men can induce inflammation and promote susceptibility to HIV infection. We will accomplish this by assessing in Aim 1 how HIV infection and ART treatment status affect the quantity, composition, and RNA cargo of exosomes in SP. Then in Aim 2, we will directly test the whether exosomes from HIV-infected men contribute towards the ability of SP to induce inflammation in cells lining the genital mucosa, and whether these exosomes can increase the susceptibility of permissive cells to HIV infection. These studies will be the first t characterize SP exosomes from HIV-infected men, and will lead to a better understanding of how these vesicles affect the early events of sexual transmission of HIV.